The long term objective of this research is to understand the origin, differentiation and lineage of murine natural killer cells. We propose to develop a stem cell assay for NK cells by utilizing adoptive transfer of bone marrow cells into lethally irradiated recipients in vivo. The surface phenotype of the NK stem cell will be characterized by a variety of antibodies including NK specific anti NKl antibodies. Possible relationship between NK stem cells and multipotential myeloid stem cells (CFUs) will be investigated by utilizing the spleen colony assay, the use of myelotoxic drug busulfan and by utilizing mutant mice deficient in myeloid stem cell function/members. The possibility that NK cells or their precursors may be related to prethymic T cells will be investigated by studying the effect of depleting bone marrow pre-T cells on NK cell regeneration in vivo. Conversely, the role of NK cells as T cell progenitors will be ascertained by investigating the effects of NK cell depletion on the T cell regeneration in vivo. The role of bone marrow stroma in NK cell differentiation will be investigated by establishing long term bone marrow cultures by the Dexter technique. The hypothesis that NK cells fail to differentiate in 89 strontium or Beta-estradiol treated mice due to destruction of bone marrow microenvironment will be tested in vivo by utilizing the Dexter culture technique. In vivo, the ability of NK precursors obtained from 89 Sr-treated/estradiol treated mice to differentiate in the marrow of bg/bg mutant mice will be tested by adoptive transfer and parabiosis. In the context of these studies we will also investigate the basis of the observed heterogeneity of NK cells. Since NK cells are considered important in immunesurveillance, an understanding of their origin and factors affecting their differentiation is important.